1. CHAPTER 9
Molecular Structure & Covalent Bonding Theories

2. Chapter Goals A Preview of the Chapter
Valence Shell Electron Pair Repulsion (VSEPR) Theory
Polar Molecules:The Influence of Molecular Geometry
Valence Bond (VB) Theory
Molecular Shapes and Bonding

3. Chapter Goals Linear Electronic Geometry: AB2 Species
Trigonal Planar Electronic Geometry: AB3 Species
Tetrahedral Electronic Geometry: AB4 Species
Tetrahedral Electronic Geometry: AB3U Species
Tetrahedral Electronic Geometry: AB2U2 Species
Tetrahedral Electronic Geometry – ABU3 Species
Trigonal Bipyramidal Geometry
Octahedral Geometry
Compounds Containing Double Bonds
Compounds Containing Triple Bonds
A Summary of Electronic and Molecular Geometries

4. Stereochemistry
Stereochemistry is the study of the three dimensional shapes of molecules.
Some questions to examine in this chapter are:
Why are we interested in shapes?
What role does molecular shape play in life?
How do we determine molecular shapes?
How do we predict molecular shapes?

5. Two Simple Theories of Covalent Bonding
Valence Shell Electron Pair Repulsion Theory
Commonly designated as VSEPR
Principal originator
R. J. Gillespie in the 1950’s
Valence Bond Theory
Involves the use of hybridized atomic orbitals
L. Pauling in the 1930’s & 40’s

6. Overview of Chapter
The same basic approach will be used in every example of molecular structure prediction:
Draw the correct Lewis dot structure.
Identify the central atom.
Designate the bonding pairs and lone pairs of electrons on central atom.
Count the regions of high electron density on the central atom.
Include both bonding and lone pairs in the counting.

7. Overview of Chapter
Determine the electronic geometry around the central atom.
VSEPR is a guide to the geometry.
Determine the molecular geometry around the central atom.
Ignore the lone pairs of electrons.
Adjust molecular geometry for effect of any lone pairs.

8. Overview of Chapter Determine the hybrid orbitals on central atom.
Repeat procedure if there is more than one central atom in molecule.
Determine molecular polarity from entire molecular geometry using electronegativity differences.

9. VSEPR Theory
Regions of high electron density around the central atom are arranged as far apart as possible to minimize repulsions.
There are five basic molecular shapes based on the number of regions of high electron density around the central atom.

10. VSEPR Theory
Two regions of high electron density around the central atom.

11. VSEPR Theory
Three regions of high electron density around the central atom.

12. VSEPR Theory
Four regions of high electron density around the central atom.

13. VSEPR Theory
Five regions of high electron density around the central atom.

14. VSEPR Theory
Six regions of high electron density around the central atom.

15. VSEPR Theory
Frequently, we will describe two geometries for each molecule.
Electronic geometry is determined by the locations of regions of high electron density around the central atom(s).
Molecular geometry determined by the arrangement of atoms around the central atom(s).
Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used.

16. VSEPR Theory
An example of a molecule that has the same electronic and molecular geometries is methane - CH4.
Electronic and molecular geometries are tetrahedral.

17. VSEPR Theory
An example of a molecule that has different electronic and molecular geometries is water - H2O.
Electronic geometry is tetrahedral.
Molecular geometry is bent or angular.

18. VSEPR Theory
Lone pairs of electrons (unshared pairs) require more volume than shared pairs.
Consequently, there is an ordering of repulsions of electrons around central atom.
Criteria for the ordering of the repulsions:

19. VSEPR Theory Lone pair to lone pair is the strongest repulsion.
Lone pair to bonding pair is intermediate repulsion.
Bonding pair to bonding pair is weakest repulsion.
Mnemonic for repulsion strengths
lp/lp > lp/bp > bp/bp
Lone pair to lone pair repulsion is why bond angles in water are less than 109.5o.

20. Polar Molecules: The Influence of Molecular Geometry
Molecular geometry affects molecular polarity.
Due to the effect of the bond dipoles and how they either cancel or reinforce each other.
A B A
A B A
linear molecule
nonpolar
angular molecule
polar

21. Polar Molecules: The Influence of Molecular Geometry
Polar Molecules must meet two requirements:
One polar bond or one lone pair of electrons on central atom.
Neither bonds nor lone pairs can be symmetrically arranged that their polarities cancel.
(Recall these from previous chapter)

22. Valence Bond (VB) Theory
Covalent bonds are formed by the overlap of atomic orbitals.
Atomic orbitals on the central atom can mix and exchange their character with other atoms in a molecule.
Process is called hybridization.
Hybrids are common:
Pink flowers
Mules
Hybrid Orbitals have the same shapes as predicted by VSEPR.

23. Valence Bond (VB) Theory
Regions of High Electron Density
Electronic Geometry
Hybridization 2
Linear sp 3
Trigonal planar
sp2 4
Tetrahedral
sp3 5
Trigonal bipyramidal
sp3d 6
Octahedral
sp3d2

24. Molecular Shapes and Bonding
In the next sections we will use the following terminology:
A = central atom
B = bonding pairs around central atom
U = lone pairs around central atom
For example:
AB3U designates that there are 3 bonding pairs and 1 lone pair around the central atom.

25. Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Some examples of molecules with this geometry are:
BeCl2, BeBr2, BeI2, HgCl2, CdCl2
All of these examples are linear, nonpolar molecules.
Important exceptions occur when the two substituents are not the same!
Be-Cl-Br or Be-I-Br will be linear and polar!

26. Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Electronic Structures
Lewis Formulas
1s 2s 2p
Be  
3s p
Cl [Ne]    

27. Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Dot Formula
Electronic Geometry

28. Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Molecular Geometry
Polarity

29. Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
1s 2s 2p
Be  
1s sp hybrid 2p
   
3s p
Cl [Ne]  

30. Linear Electronic Geometry:AB2 Species (No Lone Pairs of Electrons on A)

31. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)
Some examples of molecules with this geometry are:
BF3, BCl3
All of these examples are trigonal planar, nonpolar molecules.
Important exceptions occur when the three substituents are not the same!
BF2Cl or BCI2Br will be trigonal planar and polar!

32. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)
Electronic Structures 
Lewis Formulas
1s 2s 2p
B  
3s p
Cl [Ne] 

33. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)
Dot Formula
Electronic Geometry

34. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)
Polarity
Molecular Geometry

35. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
1s 2s 2p
B 
1s sp2 hybrid
    
3s p
Cl [Ne] 

36. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)

37. Trigonal Planar Electronic Geometry: AB3 Species (No Lone Pairs of Electrons on A)

38. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Some examples of molecules with this geometry are:
CH4, CF4, CCl4, SiH4, SiF4
All of these examples are tetrahedral, nonpolar molecules.
Important exceptions occur when the four substituents are not the same!
CF3Cl or CH2CI2 will be tetrahedral and polar!

39. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Electronic Structures
Lewis Formulas
2s p
C [He]  1s
H 

40. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Dot Formula
Electronic Geometry

41. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Molecular Geometry
Polarity

42. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
four sp3 hybrid orbitals
C [He]
2s p
C [He]   1s
H 

43. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)

44. Tetrahedral Electronic Geometry: AB4 Species (No Lone Pairs of Electrons on A)

45. Example of Molecules with More Than One Central Atom Alkanes CnH2n+2
Alkanes are hydrocarbons with the general formula CnH2n+2.
CH4 - methane
C2H6 or (H3C-CH3) - ethane
C3H8 or (H3C-CH2-CH3) - propane
The C atoms are located at the center of a tetrahedron.
Each alkane is a chain of interlocking tetrahedra.
Sufficient H atoms to form a total of four bonds for each C.

46. Example of Molecules with More Than One Central Atom Alkanes CnH2n+2

47. Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Some examples of molecules with this geometry are:
NH3, NF3, PH3, PCl3, AsH3
These molecules are our first examples of central atoms with lone pairs of electrons.
Thus, the electronic and molecular geometries are different.
All three substituents are the same but molecule is polar.
NH3 and NF3 are trigonal pyramidal, polar molecules.

48. Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Lewis Formulas
Electronic Structures
2s p
N [He] 
2s p
F [He]   1s
H 

49. Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Dot Formulas
Electronic Geometry

50. Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Molecular Geometry
Polarity

51. Tetrahedral Electronic Geometry: AB3U Species (One Lone Pair of Electrons on A)
Valence Bond Theory (Hybridization)
four sp3 hybrids
Þ ­¯ ­ ­ ­
2s p
N [He] ­¯ ­ ­ ­

52. Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Some examples of molecules with this geometry are:
H2O, OF2, H2S
These molecules are our first examples of central atoms with two lone pairs of electrons.
Thus, the electronic and molecular geometries are different.
Both substituents are the same but molecule is polar.
Molecules are angular, bent, or V-shaped and polar.

53. Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Lewis Formulas
Electronic Structures
2s p
O [He]     1s
H 

54. Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Molecular Geometry
Polarity

55. Tetrahedral Electronic Geometry: AB2U2 Species (Two Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
2s p
O [He] ­¯ ­¯ ­ ­
four sp3 hybrids
Þ ­¯ ­¯ ­ ­

56. Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Some examples of molecules with this geometry are:
HF, HCl, HBr, HI, FCl, IBr
These molecules are examples of central atoms with three lone pairs of electrons.
Again, the electronic and molecular geometries are different.
Molecules are linear and polar when the two atoms are different.
Cl2, Br2, I2 are nonpolar.

57. Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Dot Formula
Electronic Geometry

58. Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Polarity
HF is a polar molecule.
Molecular Geometry

59. Tetrahedral Electronic Geometry: ABU3 Species (Three Lone Pairs of Electrons on A)
Valence Bond Theory (Hybridization)
2s p
F [He] ­¯ ­¯ ­ ­
four sp3 hybrids
Þ ­¯ ­¯ ­ ­

60. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Some examples of molecules with this geometry are:
PF5, AsF5, PCl5, etc.
These molecules are examples of central atoms with five bonding pairs of electrons.
The electronic and molecular geometries are the same.
Molecules are trigonal bipyramidal and nonpolar when all five substituents are the same.
If the five substituents are not the same polar molecules can result, AsF4Cl is an example.

61. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Electronic Structures
Lewis Formulas
4s p
As [Ar] 3d10  
2s p
F [He]  

62. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Dot Formula
Electronic Geometry

63. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Polarity
Molecular Geometry

64. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Valence Bond Theory (Hybridization)
4s p 4d
As [Ar] 3d10   ___ ___ ___ ___ ___ ß
five sp3 d hybrids d
     ___ ___ ___ ___ ___

65. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
If lone pairs are incorporated into the trigonal bipyramidal structure, there are three possible new shapes.
One lone pair - Seesaw shape
Two lone pairs - T-shape
Three lone pairs – linear
The lone pairs occupy equatorial positions because they are 120o from two bonding pairs and 90o from the other two bonding pairs.
Results in decreased repulsions compared to lone pair in axial position.

66. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
AB4U molecules have:
trigonal bipyramid electronic geometry
seesaw shaped molecular geometry
and are polar
One example of an AB4U molecule is
SF4
Hybridization of S atom is sp3d.

67. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry

68. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
AB3U2 molecules have:
trigonal bipyramid electronic geometry
T-shaped molecular geometry
and are polar
One example of an AB3U2 molecule is
IF3
Hybridization of I atom is sp3d.

69. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry

70. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
AB2U3 molecules have:
trigonal bipyramid electronic geometry
linear molecular geometry
and are nonpolar
One example of an AB2U3 molecule is
XeF2
Hybridization of Xe atom is sp3d.

71. Trigonal Bipyramidal Electronic Geometry: AB5, AB4U, AB3U2, and AB2U3
Molecular Geometry

72. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Some examples of molecules with this geometry are:
SF6, SeF6, SCl6, etc.
These molecules are examples of central atoms with six bonding pairs of electrons.
Molecules are octahedral and nonpolar when all six substituents are the same.
If the six substituents are not the same polar molecules can result, SF5Cl is an example.

73. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Electronic Structures
Lewis Formulas
4s p
Se [Ar] 3d10  
 s p
F [He]  

74. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Polarity
Molecular Geometry

75. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Valence Bond Theory (Hybridization)
4s p d
Se [Ar] 3d10   __ __ __ __ __ ß
six sp3 d2 hybrids d
      __ __ __

76. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
If lone pairs are incorporated into the octahedral structure, there are two possible new shapes.
One lone pair - square pyramidal
Two lone pairs - square planar
The lone pairs occupy axial positions because they are 90o from four bonding pairs.
Results in decreased repulsions compared to lone pairs in equatorial positions.

77. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
AB5U molecules have:
octahedral electronic geometry
Square pyramidal molecular geometry
and are polar.
One example of an AB5U molecule is
IF5
Hybridization of I atom is sp3d2.

78. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Molecular Geometry

79. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
AB4U2 molecules have:
octahedral electronic geometry
square planar molecular geometry
and are nonpolar.
One example of an AB4U2 molecule is
XeF4
Hybridization of Xe atom is sp3d2.

80. Octahedral Electronic Geometry: AB6, AB5U, and AB4U2
Polarity
Molecular Geometry

81. Compounds Containing Double Bonds
Ethene or ethylene, C2H4, is the simplest organic compound containing a double bond.
Lewis dot formula
N = 2(8) + 4(2) = 24
A = 2(4) + 4(1) = 12
S = 12
Compound must have a double bond to obey octet rule.

82. Compounds Containing Double Bonds
Lewis Dot Formula

83. Compounds Containing Double Bonds
VSEPR Theory suggests that the C atoms are at center of trigonal planes.

84. Compounds Containing Double Bonds
VSEPR Theory suggests that the C atoms are at center of trigonal planes. H H C C H H

85. Compounds Containing Double Bonds
Valence Bond Theory (Hybridization)
C atom has four electrons.
Three electrons from each C atom are in sp2 hybrids (1 for C-C, 2 for C-H bonds) .
One electron in each C atom remains in an unhybridized p orbital
2s 2p three sp2 hybrids 2p
C  Þ   

86. Compounds Containing Double Bonds
An sp2 hybridized C atom has this shape.
Remember there will be one electron in each of the three lobes.
Top view of
an sp2 hybrid

87. Compounds Containing Double Bonds
The single 2p orbital is perpendicular to the trigonal planar sp2 lobes.
The fourth electron is in the p orbital.
Side view of sp2 hybrid
with p orbital included.

88. Compounds Containing Double Bonds
Two sp2 hybridized C atoms plus p orbitals in proper orientation to form C=C double bond.

89. Compounds Containing Double Bonds
The portion of the double bond formed from the head-on overlap of the sp2 hybrids is designated as a s bond.

90. Compounds Containing Double Bonds
The other portion of the double bond, resulting from the side-on overlap of the p orbitals, is designated as a p bond.

91. Compounds Containing Double Bonds
Thus a C=C bond looks like this and is made of two parts, one  and one  bond.

92. Compounds Containing Triple Bonds
Ethyne or acetylene, C2H2, is the simplest triple bond containing organic compound.
Lewis Dot Formula
N = 2(8) + 2(2) = 20
A = 2(4) + 2(1) =10
S = 10
Compound must have a triple bond to obey octet rule.

93. Compounds Containing Triple Bonds
Lewis Dot Formula
VSEPR Theory suggests regions of high electron density are 180o apart.
H C C H

94. Compounds Containing Triple Bonds
Valence Bond Theory (Hybridization)
Carbon has 4 electrons.
Two of the electrons are in sp hybrids.
Two electrons remain in unhybridized p orbitals.
2s 2p two sp hybrids 2p
C [He]   Þ   

95. Compounds Containing Triple Bonds
A  bond results from the head-on overlap of two sp hybrid orbitals.

96. Compounds Containing Triple Bonds
The unhybridized p orbitals form two p bonds.
Note that a triple bond consists of one  and two p bonds.

97. Compounds Containing Triple Bonds
The final result is a bond that looks like this.

98. Summary of Electronic & Molecular Geometries

99. Synthesis Question
The basic shapes that we have discussed in Chapter 8 are present in essentially all molecules. Shown below is the chemical structure of vitamin B6 phosphate. What is the shape and hybridization of each of the indicated atoms in vitamin B6 phosphate?

100. Synthesis Question

101. Group Question
Shown below is the structure of penicillin-G. What is the shape and hybridization of each of the indicated atoms in penicillin-G?

102. End of Chapter 8